In recent years, as the electric power demand has been increased, there are increasing expectations for high-voltage DC power transmission as means for implementing large-capacity/long-distance power transmission, power interchange between different frequency systems, and the like. High-voltage DC power transmission can reduce power transmission loss and also reduce the equipment cost for power transmission lines. Thus, for long-distance power transmission, high-voltage DC power transmission is more advantageous in cost aspect than alternating-current (AC) power transmission. Accordingly, high-voltage DC power transmission has been rapidly spreading domestically and abroad.
For such high-voltage DC power transmission, there is a power converter employed for converting electric power in an AC system into DC power, or for converting DC power flowing through a DC line into AC power. Although a separately-excited converter to which a thyristor is applied has been conventionally used as a power converter, it is recently considered to apply a self-excited voltage type converter.
For example, Japanese Patent Laying-Open No. 2005-094874 (PTD 1) discloses a method of operating a DC power transmission facility. According to this operating method, when a failure occurs in one of a plurality of self-excited AC/DC converters connected in series or parallel, (i) when the failed self-excited AC/DC converter is connected in series, the failed self-excited AC/DC converter is bypassed, and (ii) when the failed self-excited AC/DC converter is connected in parallel, the failed self-excited AC/DC converter is separated from the DC system. Thus, the operation is continued using a sound self-excited AC/DC converter.